This invention relates to a motorized power steering apparatus for automobiles or other vehicles. In the past, power steering apparatuses for automobiles were usually driven by hydraulic power generated by a pump. However, as hydraulic systems are bulky and heavy, in recent years, there has been a trend towards the use of electric motors to provide the drive force for power steering. A power steering apparatus which employs an electric motor to generate an auxiliary torque to assist the steering of the vehicle is referred to as a motorized power steering apparatus.
In a motorized power steering apparatus, a torque sensor measures the steering torque applied by the driver to a steering wheel. An electric motor which is connected to a suitable portion of the steering gear is then controlled in accordance with the measured torque to impart an auxiliary steering force to the steering gear. The auxiliary steering force lessens the force which need by applied to the steering wheel by the driver.
Many conventional power steering apparatuses have a triangular wave generator having a prescribed oscillatory frequency. Open-loop proportional control is performed in which the pulse width of the output of the triangular wave generator is modulated by the output voltage of a torque sensor, and the pulse width modulated signal is input to a motor as a drive signal.
However, in a conventional apparatus which performs open-loop proportional control of the voltage applied to the motor, only the duty cycle of the pulses applied to the motor is controlled, and a battery is used as a voltage supply. The power steering apparatus is often connected to the battery by wiring which produces a large voltage drop when a large current flows through it. Therefore, even if the duty cycle is maintained constant, as the voltage of the power supply can vary, it is difficult to apply a voltage to the motor which is proportional to the output voltage of the torque sensor. When the impedance of the voltage supply is large, there is a considerable possibility of harmful abnormal oscillations being generated.
Furthermore, conventionally a single potentiometer generates torque signals for turns to both the left and right, so when the potentiometer itself generates noise or when there are bad contacts in the wiring or short circuits, a torque signal may be generated in the direction opposite to the direction in which the driver wishes to turn the vehicle, so there is the danger of the power steering apparatus turning the vehicle wheels in the wrong direction.